1. Field of the Invention
The present invention relates to a screw rotor assembly for a screw compressor or a similar apparatus such as a vacuum pump or an expander.
2. Description of the Prior Art
Various profiles for screw rotors for screw compressors or the like have been proposed, for example, in Japanese Patent Laid-open Nos. 59-196988 and 61-190184, and Japanese Patent Publication No. 60-41238.
A profile proposed in Japanese Patent Publication No. 60-41238 is shown in FIG. 12 by way of example. The profile is formed of a plurality of sectional curves. FIG. 12 shows the intermeshing portions of a male rotor 11 and a female rotor 12. A male rotor tooth 11a consists of sectional curves a-b, b-c, c-d, d-e, e-f and f-g successively arranged in that order from the leading side to the trailing side, and the female rotor tooth 12a consists of sectional curves A-B, B-C, C-D, D-E, E-F, F-G and G-H successively arranged in that order from the leading side to the trailing side. In FIG. 12, indicated at P.sub.M and P.sub.F are the respective pitch circles of the male rotor 11 and the female rotor 12, at A.sub.M and A.sub.F are the respective tip circles of the male rotor 11 and the female rotor 12, and at D.sub.M and D.sub.F are the respective root circles of the male rotor 11 and the female rotor 12.
Generally, the performance of the profile of the screw rotor is dependent mostly on the length of the sealing line and the area of the blow hole, and the performance is improved as both the length of sealing line and the area of blow hole are reduced. However, since a portion of a conventional profile corresponding to the curve e-f of the male rotor tooth 11a of the male rotor 11 is an arc of a circle, when either the length of the seal line or the area of blow hole is decreased, the other increases as indicated by a curve I in FIG. 13, and hence it has been impossible to reduce both the length of the sealing line and the area of the blow hole. Furthermore, since a portion of the conventional profile corresponding to the curve C-E of the female rotor tooth 12a of the female rotor 12 is formed of two curves respectively defined by two functions, the length of the sealing line is increased inevitably. That is, the respective quadratic derivatives of those functions at the junction of the curves do not coincide with each other. Therefore, a line indicating the variation of the length L of the sealing line in the xy plane with the rotating angle .psi. is bent at the junction, and thereby the length of the sealing line is increased as indicated by a broken line I in FIG. 7.